1. Field of the Invention
The present invention relates generally to a Push-to-talk over Cellular (PoC) terminal, and more particularly to a method for informing a PoC user of remaining time information available for media transmission when a communication has been established according to a request for the communication.
2. Description of the Related Art
With the rapid development of mobile communications and the vast expansion of communication networks, cellular phones are now being used to provide a wider variety of extended services and applications. Cellular phone users are requiring a wider variety of services beyond the basic talk communication service, such as location, multimedia and Push-To-Talk (PTT) services. The current PTT service supports various additional functions, such as instant messenger and status display functions, as well as group and voice communication, which may also be provided by an existing radio transmitter or a Trunk Radio System (TRS).
Active research currently being conducted about establishment of a standard for a PoC service to be provided through a mobile communication network using the PTT concept. The PoC service is distinguished from the existing mobile communication services, in that a user belongs to multiple sessions and thus can continue communication while moving between sessions if necessary. The requirement that a user be capable of continuing communication while moving between sessions is clearly found in a requirement of the Open Mobile Alliance (OMA), which is a forum for specifying mobile communications services.
FIG. 1 illustrates a basic structure of a conventional PoC system. Referring to FIG. 1, the PoC client 10 corresponds to a service requester embedded in a mobile terminal, which is usually connected through an access network 20 to an Session Initiation Protocol (SIP) and Internet Protocol (IP) core network 30, which is for supporting SIP/IP multimedia.
The PoC client 10 resides in a PoC client terminal in order to enable access to a PoC service. A main functions of the PoC client 10 are to generate a PoC session at the PoC client side, and participate in and terminate the session. Moreover, the PoC client 10 functions to make and transfer a talk burst, support an instant personal alert and perform authentication when accessing the PoC service. Hereinafter, unless otherwise stated, the PoC client 10 is assumed to be the same as a PoC service subscriber.
The SIP/IP core network 30 is connected to a PoC server 60, a Group List and Management System (GLMS) 50 and a presence server 70, in order to support a PoC service.
The GLMS 50 is a server that stores information of PoC service subscribers and transfers subscribers (e.g., subscriber ID, terminal number and group information) in cooperation with the PoC server 60.
The presence server 70 stores status information, such as location and status (i.e., on-line or off-line) of a PoC service subscriber, which registers/maintains a current position of an on-line user and informs users of such registered/maintained information, thereby enabling instant communication between users.
The PoC server 60 serves as a Controlling PoC Function (CF) for maintaining and managing a PoC session, or a Participating PoC Function (PF) for participating in a PoC session for a one-to-one PoC call or a one-to-many PoC call (or group PoC call).
The PoC server is classified into a Controlling PoC Function (CF) of overseeing maintenance and management of a PoC session and a Participating PoC Function (PF) of overseeing maintenance and management between each PoC session, which will be explained with reference to the Tables 1 and 2 below.
TABLE 1Controlling PoC Function(CF)Provides centralized PoC session handlingProvides the centralized Media distributionProvides the centralized Talk Burst Arbitration functionalityincluding talker identificationProvides SIP session handling, such as SIP session origination,termination, etc.Provides policy enforcement for participation in group sessionsProvides the participants information Collects and providescentralized media quality informationProvides centralized charging reportsMay provide transcoding between different codecs; Support TalkBurst Control Protocol Negotiation
As shown in Table 1, the CF maintains and manages an entire PoC session. The PoC server receives requests for a floor from PoC clients, arranges an order in which to give the clients the floor, and gives the clients the floor in that order. The PoC server also distributes a talk burst, for which an arbitrary PoC client makes a request, to all other PoC clients participating in a group PoC call, and provides information of the PoC clients participating in the group PoC call.
As shown in Table 2 below, the PF manages a PoC session between the CF and each PoC client. In particular, the PF relays the floor between the PoC client and the CF when the PoC client makes a request for the floor or when the CF gives the floor to the PoC client. In addition, the PF relays media between the CF and the PoC client, performs transcoding between different codecs, and filters one of two concurrent PoC sessions according to the choice of a PoC user when there is simultaneous talking in the two concurrent PoC sessions.
TABLE 2Participating PoC Function(PF)Provides PoC session handlingMay provide the Media relay function between PoC Client andControlling PoC serverMay provide user media adaptation proceduresMay provide the Talk Burst control message relay function betweenPoC Client and Controlling PoC server Provides SIP session handling,such as SIP session origination, termination, etc, on behalf of therepresented PoC Client.Provides policy enforcement for incoming PoC session (e.g. accesscontrol, incoming PoC session barring, availability status, etc)May collect and provide media quality information; Provides theparticipant charging reportsMay provide filtering of the media streams in the case of simultaneoussessionsMay provide transcodmg between different codecsMay support Talk Burst Control Protocol Negotiation; Stores thecurrent AnswerMode and Incoming PoC Session Barring preferences of the PoC Client
In the PoC service system as described above, the PoC user can input information on a group and its members to the GLMS 50 through his/her PoC terminal, and can learn information about PoC users whom he or she can call through an individual or group list transmitted from the GLMS 50. Alternatively, the information on the group and its members may be input, corrected and managed in the GLMS 50 via a reliable communication network such as the Internet or Intranet.
In order to make use of the PoC service, the PoC user registers his/her PoC address with the SIP/IP core network 30. The SIP/IP core network 30 stores PoC user information based on the request of the PoC user. Thus, when another PoC user tries to request a group PoC call, the PoC user registers his/her information in the SIP/IP core network 30 in advance as described above, and requests the group PoC call to his/her SIP/IP core network 30 by using group identification information transmitted from the GLMS 50. At this time, the SIP/IP core network 30 performs address and domain location determination by using information of the call requesting PoC user and then transfers a PoC call request to a home PoC server with which the call requesting PoC user is registered. In regard to the PoC call request, the PoC server prepares for establishment of a PoC session, obtains each user's information from the GLMS 50, and then transfers a PoC call request signal to a corresponding SIP/IP core network 30. Here, in the case of a PoC call request to users within an Intradomain, the PoC server performs both the CF and PF. The PoC server, which manages a call-requested PoC user, requests a PoC call to the PoC user after the SIP/IP core network 30 performs the location determination procedure, by using information of the PoC user that is transmitted to the PoC server.
Hereinafter, a general procedure where a PoC user obtains a floor in a conventional PoC system having the construction described above will be discussed with reference to FIG. 2.
Within a PoC session where a PoC client A is connected to a PoC client B, when there is an input from a user in order to obtain a floor, the PoC client A makes a request for the floor by transmitting a message making a request for media transmission, a Media Burst Request message, to a PoC server A in step 200. The PoC server A acts as the PF. After receiving Media Burst Request message, the PoC server A transmits the Media Burst Request message to a PoC server X in step 202. The PoC server X acts as the CF. After receiving the message requesting the floor, the PoC server X transmits a message granting the floor, a Media Burst granted message, to the PoC client A through steps 204 and 206. Further, the PoC server X transmits a message indicating that the PoC client A has been granted the floor, a Media Burst Taken message, to the PoC client B. At this time, the Media Burst Taken message includes an IDentifier (ID) of the PoC client A, that is, information by which the PoC client B having received the message can know the identity of the talker. In the PoC communication as described above, when the talker has been granted the right to transmit the media burst, the talker can transmit media only during a time interval set by a service provider. According to a standard arranged by the Open Mobile Alliance (OMA), a service provider can determine such a time interval. The reason why a client sets and operates an available time interval for media transmission as described above is to mediate between PoC participants in a PoC system, thereby preventing exclusive and continuous media transmission by one PoC session participant, which may disturb media transmission by another PoC session participant because the PoC service employs a half-duplex scheme.
Hereinafter, a signal flow between a PoC client and a PoC server when a PoC terminal apparatus transmits a media burst in a conventional PoC system will be discussed with reference to FIG. 6. Referring to FIG. 6, the PoC client makes a request for media transmission by transmitting a media burst request message for media transmission to a PoC server in step 601 as in steps 200 to 202 of FIG. 2.
Then, the PoC server having received the media burst request message transmits a media burst granted message to the PoC client who transmitted the media burst request message in step 602 as in steps 204 to 206 of FIG. 2. Thereafter, the PoC server operates a timer T2 related to the media transmission time. When the time set in the timer T2 has expired, the PoC server transmits a media burst revoke message to the PoC client in step 603. The transmitted media burst revoke message corresponds to a message reporting that the transmission time has expired and it is impossible to continue transmitting media. After receiving the media burst revoke message, the PoC client stops encoding of the PoC user's voice, generates a media burst release message and transmits the generated media burst release message to the PoC server.
Thereafter, the PoC server operates another timer T3 for allowing extra time for transmission of the media burst remaining in the PoC client buffer message and allows the PoC client to transmit the media burst stored in the buffer while the timer T3 is operated. Further, when the PoC client has transmitted a media burst release message in step 604, the PoC server transmits a media burst idle or media burst taken message (step 605) to all PoC clients participating in the PoC session, thereby reporting that the previous PoC client has completed the media transmission. The media burst includes voice in the OMA spec PoC v1 and includes media in the OMA spec PoC v2. In the following description, the media burst includes voice. Therefore, one of the media burst may be a talk burst.
In the process described above, the media burst granted message transmitted from the PoC server X (CF X) to the PoC client A includes a header having a media transmission time value, from which the PoC client A can recognize an allowed initial media transmission time value. However, although the PoC client is aware of the initial media transmission time value, when the PoC client has transmitted the media burst during a time interval, the PoC client cannot inform a PoC user of the remaining available time for media transmission because the PoC client does not have an interval function for measuring the transmission time. Therefore, the media transmission may be unexpectedly interrupted due to the expiration of the transmission time.